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they told me that too but that stuff is expensive. i just put regular dry sand in my tank. looks cleaner to me and its easier to get the left over shells out with a net because the sand is so fine it strains right through the net. the reason its better is because it already has bacteria in it .
Bag live sand is a waste of money. Just fill your tank with dead sand and then see if you can add some sand from an established tank. Live sand is often sold on craigslist. Live rock will also seed the sand.
You can get 50lb bags of Pavestone brand crushed limestone for $5 apiece at Home Depot. It's white, looks great, and the contents are listed on the bag, calcium carbonate, magnesium, etc. It ranges from very fine to medium grains, good blend. Just make sure you don't stir it up when you add water to the tank or it will take days to clear up.
The main idea is that sand is simply the substrate in which beneficial organisms can live. whether they are present in the sand or not is what separates "live" sand from "dead."
Therefore if you add a small population of that life to a large amount of "dead" substrate, it will reproduce and expand to inhabitat the new territory, thus your sand will slowly begin to "live." Since all you need is a small amount of life to "seed" a large amount of "dead" sand, the notion of buying those big bags of live sand is largely thought of as a waste of money.
you can buy some of the "dead" substrate materials like dry crushed aragonite or limestone, as AM pointed out, and then there are many ways to "seed" it: good live rock, a cup of live sand from an established tank, or even a rag soaked in the fuge for a couple days or some old activated charcoal that has been used in a well established tank.